Which of these numbers is composite? ${3,\ 17,\ 23,\ 63,\ 73}$
Explanation: A composite number is a number that has more than two factors (including 1 and itself). 3, 17, 23, and 73 each have only two factors. The factors of 63 are 1, 3, 7, 9, 21, and 63. Thus, 63 is the composite number.